To estimate a maximum purchase price for the coin in question, we can analyze the provided data and make some comparisons.

1. Current coin data:
- Starting price: 6000 €
- Grade: 85 (ebc-)

2. Historical auction data:
- The coin has been auctioned 5 times and has sold on all occasions.
- The average starting price in previous auctions is 7800 €.
- The average hammer price in previous auctions is 12660 €.

3. Previous prices of the same coin:
- Auction 1 (Grade 85): Hammer price 10000 €.
- Auction 2 (Grade 90): Hammer price 16000 €.
- Auction 3 (Grade 95): Hammer price 16800 €.
- Auction 4 (Grade 90): Hammer price 11750 €.

4. Analysis:
- The grade of the current coin (85) is similar to that of the first auction (also 85), where the hammer price was 10000 €.
- Coins with better grades (90 and 95) have reached significantly higher hammer prices (16000 € and 16800 €).
- Given that the current coin has a grade of 85, it is reasonable to expect that its hammer price will be in a range below 10000 €.

5. Estimation:
- Considering the starting price of 6000 € and the hammer price of 10000 € in the previous auction with the same grade, an estimated maximum purchase price could be around 9000 € to 10000 €, as this price reflects the demand and the grade of the coin.

Therefore, an estimated maximum purchase price for the coin in question could be approximately 9000 €.